5of8A stranded passenger looks inside the locked entrance to the Rockridge BART station in Oakland, Calif. on Saturday, March 9, 2019 after computer glitches forced a shutdown of the entire transit system.Photo: Paul Chinn, The Chronicle

6of8The Rockridge BART station remains closed in Oakland, Calif. on Saturday, March 9, 2019 after computer glitches forced a shutdown of the entire transit system.Photo: Paul Chinn, The Chronicle

7of8Stranded passengers stand at the entrance to the Rockridge BART station in Oakland, Calif. on Saturday, March 9, 2019 after computer glitches forced a shutdown of the entire transit system.Photo: Paul Chinn, The Chronicle

8of8The Rockridge BART station platform is deserted in Oakland, Calif. on Saturday, March 9, 2019 after computer glitches forced a shutdown of the entire transit system.Photo: Paul Chinn, The Chronicle

Service wasn’t fully restored until 11 a.m. — nearly five hours after the shutdown was reported — and only limited BART service was available as of about 9:10 a.m.

The cause of the shutdown remains unknown.

Initially, BART said its closure was because of a computer network failure caused by maintenance crews in Oakland that affected the system’s traction power and rotating systems. The traction power sends energy to the trains, while the rotating system programs the direction and routes.

BART engineers were conducting a “comprehensive forensic analysis evaluation” to determine what caused the system failure, the agency said in a statement. The analysis is expected to take several days.

Janice Li, a BART board member, said the issue can be partly attributed to the age of the system. BART, which opened in 1972, has had a few upgrades, but most of the “actual infrastructure” is old, she said.

Some parts of the agency’s outdated system — the cable and train control system — will be improved through funding provided by the Measure RR bond, which was approved by voters in 2016.

But until then, Li said she wants BART management to be held accountable and ensure that system failures as expansive as Saturday’s debacle don’t happen again.

It could have been worse, she said.

“We are almost fortunate this happened on a Saturday,” Li said. “Imagine this happening during rush hour, during a weekday when we know we are servicing 420,000-plus trips a day.”

The ridership for Saturday, March 2, was 143,880 people. There is no breakdown on what the ridership was from 6 to 9 a.m., but it is “obviously a small fraction of that,” said Alicia Trost, a BART spokeswoman.

Other transit agencies, including San Francisco’s Muni, provided mutual aid service for riders early Saturday. The BART closure didn’t have any major impacts on Muni services, said Paul Rose, a spokesman.

The last time BART had a significant systemwide closure was 15 years ago, said Bevan Dufty, a BART board director.

“This is not a frequent occurrence,” he said.

The transit system’s assistant general manager of operations will give a report Thursday at the BART board meeting.

“Right now, there is still a lot of unanswered questions as to how this happened,” Dufty said.

Sarah Ravani covers Oakland and the East Bay at The San Francisco Chronicle. She joined The Chronicle in 2016 after graduating from Columbia University's Graduate School of Journalism. Previously, she covered breaking news and crime for The Chronicle. She has provided coverage on wildfires, mass shootings, the fatal shooting of police officers and massive floods in the North Bay.